The literature describes several ways of removing sulfur from petroleum streams, which fall into two primary categories: (a) catalytic hydrotreating processes in which the feed is passed over a hydrotreating catalyst at elevated temperatures and hydrogen pressures and (b) non-hydrotreating processes. The present invention is an improved non-hydrotreating process.
Non-hydrotreating processes can be further divided into those that remove the entire sulfur-bearing molecule from the feed by adsorption onto a selective adsorbent, as exemplified by Khare (U.S. Pat. No. 6,274,533; U.S. Pat. No. 6,338,794 and U.S. Pat. No. 6,482,314), and those that remove only the sulfur by chemical reaction. The latter can further be divided into those that are oxidative in nature, as exemplified by Rappas (U.S. Pat. No. 6,402,940), Ohsol, et al (U.S. Pat. No. 5,985,137; U.S. Pat. No. 5,948,242), Yen (U.S. Pat. No. 6,402,939) and Gunnerman (U.S. Pat. No. 6,500,219), and those that are reducing in nature. The most important of the reducing processes is reaction of a feed containing organosulfur species with elemental sodium as exemplified by Brons, et al (U.S. Pat. No. 6,210,564), Baird (U.S. Pat. No. 4,003,824; U.S. Pat. No. 4,123,350) and Bearden (U.S. Pat. No. 3,787,315; U.S. Pat. No. 3,788,978; U.S. Pat. No. 3,791,966; U.S. Pat. No. 3,976,559 and U.S. Pat. No. 4,076,613).
In order to contact elemental sodium with feed, the sodium is typically melted (m.p.=97.8° C.) and added to the feed as a dispersion of small droplets. Even with intense mixing, it is not possible to produce extremely small droplet sizes. As a result, sodium on the surface of the droplets may react with sulfur in the feed to form a skin of sodium sulfide (Na2S). This skin remains on the droplet exterior surface, and as Na2S has a melting point of 1,180° C., it remains as a solid under the conditions of desulfurization. The skin thereby inhibits further reaction between the sodium in the interior of the droplet and sulfur in the feed. For these reasons, it has been necessary in the prior art to use substantially higher ratios of sodium to feed sulfur than are stoichiometrically required in order to remove sulfur to the desired level.